This invention relates generally to photodiodes, and more particularly to a photodiode with improved distributed absorption capabilities.
Waveguide photodiodes are used in the digital analog and RF optical links of communication systems to convert optical power into photocurrent. Waveguide photodiodes which operate at high optical power and at a high frequency and do so efficiently within the linear range of the photodiode are critical to a high powered optical link. These high power, high efficiency photodiodes are necessary to provide a communication system which has reduced RF insertion loss, increase signal-to-noise ratio and increased linearity for the communication system.
Referring to FIG. 1, a typical waveguide photodiode 10 includes a light absorption layer 12 sandwiched between a top cladding layer 14 and a bottom cladding layer 16. Light 17 is directed into the absorption layer 12 through one side of the photodiode 10. The cladding layers 14, 16 are configured to guide the light 17 in the photodiode 10 in a preselected optical mode. Two contact layers 18, 20 are positioned on opposite sides of the cladding layers 14, 16 and are positively and negatively polarized, respectively, by a voltage supply (not shown). The light 17 propagates into the absorption layer 12 which absorbs the light 17 and photogenerates carriers therefrom in the form of pairs of positively charged holes and negatively charged electrons. The voltage supply produces an electric field which provides the stimulus to cause the positively charged holes to travel towards the negatively charged contact layer 16, and the negatively charged electrons to travel towards the positively charged contact layer 14. The electric field strength inside the absorption layer 12 of a photodiode 10 determines the speed at which the holes and electrons travel to their respective contact layers, it is therefore very important to maintain a high electric field strength level over the entire length, x, of the photodiode 10 in order to provide an efficient photodiode 10. The contact layers 14,16 collect the electrons and holes that form the photocurrent.
One measure of the performance of a photodiode 10 is the efficiency. Efficiency of a photodiode is a measure of how much of the input light signal is photogenerated into carriers that subsequently form the photocurrent. Other measures of the performance of an RF optical link are the gain and noise figure of the link. High gain and low noise figure are very important to an analog RF optical link of a communications system using the photodiode 10, particularly in those links which have external modulation. The gain of the RF optical link is proportional to the square of the photocurrent and the noise figure is inversely proportional to the photocurrent making both the gain and the noise figure dependant on the efficiency of the photodiode 10. Thus, it is desirable to maximize the efficiency of a photodiode 10 to maximize the gain and minimize the noise figure.
To do so in a typical Indium-Phosphate (InP) photodiode 10 operating in the 1.3-1.55 .mu.m wavelength range, the absorption layer 12 is formed of an InGaAs material which is lattice-matched to InP. InGaAs is typically used for the absorption layer 12 because of its superior light absorbing properties. The absorption coefficient (.alpha.) is a measure of the light absorbing capability of a material. For InGaAs, the .alpha. is approximately 0.8-1.0 .mu.m.sup.-1 measured at a wavelength of 1.55 .mu.m.
An absorption layer 12 having a high absorption coefficient such as 0.8-1.0 .mu.m.sup.-1 provides for good absorption of the input light signal 17; however, as shown in FIG. 2, the absorption and generation of photocurrent occurs primarily in the first few micrometers of the photodiode 10 (FIG. 1). This results in a large percentage of the carriers being photo-generated in the first few micrometers (.mu.m) of the absorption layer 12 which can result in an electric field collapse in the beginning region of the absorption layer 12.
The collapse of the electric field has the undesirable effect of reducing the velocity of the carriers and inhibiting the collection of the carriers by the contact layers 14, 16. In addition, photogenerating a large number of carriers in the beginning region of the photodiode 10 has the undesirable effect of increasing the harmonic frequency generated by the photodiode 10 as well as generating a large amount of heat in the beginning portion of the photodiode 10. This can lead to a thermal failure of a communications link which contains the photodiode 10.
To solve this problem, photodiodes which distribute the absorption of light in a more uniform manner across the length (x) of the photodiode 10 have been developed. One such photodiode is the velocity-matched photodiode 30 shown in FIG. 3. The velocity matched photodiode includes a non-absorbing waveguide core 32 which is sandwiched between two cladding layers 34, 35. Coplanar strips 40 are positioned on top of one of the cladding layers 34. Several interdigitated MSM photodiodes 36 are placed on top of one of the cladding layers 34 and are coupled together through an optical waveguide 38. Additional information on interdigitated MSM photodiodes 36 can be found at "InGaAs Metal-Semiconductor-Metal Photodetectors for Long Wavelength Optical Communications," by J. B. D. Stoole, et al., IEEE J. of Quantum Elec., Vol. 27, No. 3, March 1991.
The cladding layers 34, 35 guide the light 37 in the photodiode 30. Each MSM photodiode 36 contains an absorbing layer and is operative to couple a portion of the light 37 propagating in the photodiode 30 into the absorbing layer within each of the MSM photodiode 36.
The MSM photodiodes 36 generate photocurrent from the coupled light.
Since only a portion of the light 37 is converted into photocurrent by each MSM photodiode 36, the absorption of the light 37 is spread over the length (x) of the photodiode 30. The MSM photodiodes 36 are configured and positioned in preselected locations across the length, x, of the photodiode 30 so as to match the group velocity of the light 37 in the waveguide core 32 with the group velocity of the photocurrent microwave signal in the coplanar strips 40. This matching of velocities is needed for the light absorbed by each MSM photodiode 36 to be added together in phase in the optical waveguide 38. To have high efficiency, a high number of MSM photodiodes is required. This makes the velocity-matched photodiode very long.
In addition, the velocity-matched photodiode 30 can be difficult to fabricate since electron-beam lithography is generally required to pattern the MSM photodiodes 36. It can also be lossy since the velocity-matched photodiode 30 requires an impedance coupled to the MSM photodiodes 36 to terminate the coplanar waveguide 38. This is undesirable since a termination impedance reduces the amount of photocurrent delivered to the next stage in the communications link which is typically an amplifier.
Referring to FIG. 4, another photodiode which distributes the absorption of light 48 in a more uniform manner across the length, x, of the photodiode 50 is an expanded optical mode photodiode 50. Referring to FIGS. 4 & 5, a comparison of a side sectional view of a portion of the typical prior art photodiode 10 of FIG. 1 and a sectional view of a portion of the expanded mode photodiode 50 shows that the expanded optical mode photodiode 50 provides a thinner absorbing layer 52 and thicker cladding layers 54, 55 than that provided by the typical photodiode 10. In the typical photodiode 10, the optical mode 56 occurs mostly in the absorbing layer 12 whereas in the expanded optical mode photodiode 50, the optical mode 58 occurs mostly in the cladding layers 54, 55 which do not absorb light. Thus, a smaller amount of the light 48 is absorbed per unit length, x, in the expanded optical mode photodiode 50 providing for a more uniform absorption of the light 48 across the length, x, of the photodiode 50. This results in the generated heat being more uniformly dissipated across the length, x, of the photodiode 50 and reduces the amount of heat dissipated per unit length in the photodiode 50. However, the expanded optical mode photodiode 50 still provides for high carrier density in the beginning portion of the photodiode 50 (around x=0) which can result in non-linear effects in the response of the photodiode.
What is needed therefore is a photodiode which would reduce the amount of heat dissipated per unit length and also reduce the amount of carriers being photogenerated in the beginning portion of the photodiode while at the same time, providing an efficient photodiode.